Printed circuit boards often are found to contain errors, or require changes due to improvements in the circuits. The circuit board changes can be made by changing the art work for the board. However, this can be very expensive. Even if the changes are fairly extensive, often it is far less costly to hand-apply jumper wires in selected locations on the board. The new connections usually are made by inserting one bare end of an insulated jumper wire into a plated hole in the board, gluing the wire onto the surface of the board along a predetermined path, and inserting the other bare end into another plated hole. Then the board is passed through a wave soldering bath which solders all of the wires and component terminals in the plated holes. Alternatively, the terminals can be hand-soldered in place.
The board surfaces onto which the jumper wires are glued most often are coatings of "solder resist" materials, usually epoxy resins. The adhesives used to glue the jumper wires onto those surfaces vary; they include silicone rubber, acrylics epoxies, and hot-melt adhesives. The adhesives are applied manually. The prior methods of applying adhesives suffer various disadvantages. For example, the wire usually must be held in place for a substantial amount of time to allow for sufficient curing or cooling of the adhesive to properly secure the wire to the printed circuit board. Usually this is a time-consuming process and may result in a poor bond if sufficient cooling or curing time is not allocated.
Another disadvantage of prior methods is that they are messy. This increases labor costs for the process due to increased set-up and clean-up time. Also, the prior methods usually are awkward to use. This further reduces worker productivity.
When epoxy adhesives are used, the two components are mixed in a batch which is kept near the work station where it is to be used. The material hardens into a useless mass if it is not used promptly; that is, the material does not have a very long "pot life". This is wasteful, costly and requires additional labor to mix new batches relatively frequently.
Some prior adhesives are toxic or give off toxic fumes when used, thus posing a health hazard to workers and necessitating the use of special safety procedures and/or equipment.
In accordance with one feature of the present invention, a heating tool is used to melt a hot-melt adhesive on a wire to adhere the wire to circuit element surfaces. This creates the new problem that the heating tool should be cleaned frequently in order to prevent the build-up of hardened adhesive on it. Cloths can be used for this purpose, but they are relatively slow and inefficient to use since they must be replaced frequently as the surface becomes covered with waste adhesive, and since a clean area on the cloth may take a considerable length of time to find. Also, the cloth may smolder or burn due to contact with the iron.
A cleaning problem similar to that experienced with hot-melt adhesive melting irons is found in using soldering irons. Wet sponges usually are used to wipe the excess solder and flux materials off of the iron tip. However, the sponge dries out and becomes charred and must be replaced relatively frequently.
A problem with some jumper wires which have thin coatings of high-temperature-resistant plastics such as polytetrafluoroethylene (e.g. DuPont "Teflon") for insulation is that they are not very resistant to cuts and abrasion. This endangers the integrity of the circuits in which they are used.